tests/basics: Split f-string debug printing to separate file with .exp.

This feature {x=} was introduced in Python 3.8 so needs a separate .exp
file to run on earlier Python versions.

See https://bugs.python.org/issue36817

Signed-off-by: Damien George <damien@micropython.org>

tests/basics/string_fstring_debug.py

@@ -0,0 +1,23 @@
+# test f-string debug feature {x=}
+
+
+def f():
+    return 4
+
+
+def g(_):
+    return 5
+
+
+def h():
+    return 6
+
+
+x, y = 1, 2
+print(f"{x=}")
+print(f"{x=:08x}")
+print(f"a {x=} b {y} c")
+print(f"a {x=:08x} b {y} c")
+
+print(f'a {f() + g("foo") + h()=} b')
+print(f'a {f() + g("foo") + h()=:08x} b')